Zinc sulfide electroluminophores and method for production thereof

ABSTRACT

Zinc sulfide electroluminophores are prepared from solutions of zinc salts with hydrogen sulfide. The zinc sulfide compounds are mixed with activator and coactivator compounds to produce luminophores, and the mixtures are fired in the presence of fixing agents. These fired materials are then treated in an acid bath, washed, neutralized, and optionally filtered and dried.

REFERENCE TO RELATED APPLICATIONS

The present application is the national stage under 35 U.S.C. §371 ofinternational application PCT/EP00/11069, filed Nov. 9, 2000 whichdesignated the United States, and which application was not published inthe English language.

The present invention relates to zinc sulfide (ZnS) based fine-grainelectroluminescent phosphors and methods for their production.

Phosphors of this type are generally doped with copper (Cu), however,optionally also with copper and/or gold (Au), as well as with copper andmanganese (Mn) and they furthermore contain one or more coactivators,for which purpose halide anions (Cl, Br, I) or certain tervalent cations(e.g., Al, Ga, In) are usually inserted into the ZnS lattice.

In dependence upon their specific chemical composition, the zinc sulfideelectroluminophores emit in the blue, green or yellow-orange range ofthe visible spectrum. In technical applications they are used tomanufacture electroluminescent lamps that are used either for thebackground lighting of LCD displays (clocks, pocket calculators, mobiletelephones, instrument lighting, etc.) or as luminous and markingelements, e.g., in airplanes and motor vehicles, in the interior and onthe facades of buildings, as well as in advertising fixtures, etc.

It is known that zinc sulfide electroluminophores, as compared totechnical luminescent substances for the LTV, X-ray and cathode rayexcitation, have a comparatively short life. The half-life (which is thetime in which the brightness of the EL phosphors decreases to half itsoriginal value) of unencapsulated electroluminophores is only a fewhundred hours. It is widely believed that their service life issignificantly influenced, among other factors, by the grain size of theelectroluminophores. This is one of the reasons why commerciallyavailable ZnS EL phosphors typically have average grain sizes in therange of 20 to 40 μm.

However, the extent to which pigments of such a large size can be madeinto high-quality layers is limited. In the commonly used screenprinting techniques, for example, they require the use of comparativelylarge screens, resulting in dry layer thicknesses of up to 40 μm. Theseoften display a visibly uneven EL emission due to the unavoidableinhomogeneities of such layers.

Another shortcoming of the resulting thick electroluminescentarrangements that is attributable to the large grain size ofcommercially available EL pigments lies in the fact that relatively highsupply voltages are required to attain the desired levels of brightness.These can result in high stresses on the incorporated binding agents andthus in a reduction in the service life of the EL arrangements.

Moreover, with the use of the coarse EL pigments according to the priorart, it is possible that individual particularly large luminophoreparticles can protrude from the layer despite the adjusted layerthickness of up to 40 μm. In these cases the voltage stability and/orelectric strength of the EL films is reduced, resulting in an additionalreduction in their service life.

A significant reduction in the average grain size of the EL phosphorsand simultaneous preservation or improvement of the brightness anddurability values is therefore highly desirable for many technicalapplications that are based on the use of screen printing processes.

If, as recently proposed in DE 19 708 543, EL pigments are even to beprocessed into fine graphic structures, such as security elements in thefield of value product printing by means of intaglio or offset printingprocesses, the availability of fine-grain EL phosphors must beconsidered an essential prerequisite for the technical feasibility ofsuch an application. Experience has shown that it is necessary in thiscase to use average pigment grain sizes of 2 to 6 gm to be able to meetthe technical requirements of these printing processes.

Methods for producing efficient EL phosphors have been known for a longtime. The pertinent prior art is described, for example, in U.S. Pat.No. 4,859,361 and in WO 91/16722. According to those patent documents,the following steps are required to produce Cu doped or Cu and Mn dopedZnS electroluminophores that are coded with the usual coactivators:

Step 1: Preparation of a mixture of ZnS, the desired quantity of an ELactivator (e.g., CuSO₄) and a coactivating, halide-containing fluxingagent (usually BaCl₂, MgCl₂, NaCl).

Step 2: Firing of this mixture at temperatures between 1000 and 1300° C.

Step 3: Cooling of the fired material to room temperature and rinsingwith water.

Step 4: Mechanical working of the material by milling.

Step 5: Renewed firing of the thusly treated material in the temperaturerange between 600 and 900° C., optionally after previous renewedaddition of ZnSO₄ and CuSO₄.

Step 6: Cooling to room temperature and optional quenching with H₂Oafter a certain cooling time.

Step 7: Optional washing with H₂O and/or mineral acids to remove solublecomponents and with KCN solution to remove excess Cu₂S.

Particular importance is attached by the invention to the 4^(th)preparation step. The mechanical working of the material that was firstfired at 1000 to 1300° C. is intended to transform a portion of thehexagonal ZnS electroluminophore formed under these conditions into thecubical crystal form. It is alleged that a transformation of this typeeffects an improvement of the brightness of the EL phosphors andparticularly increases their life. When the described process andcomparable process variations are used, zinc sulfide electroluminophoreswith average grain sizes between 20 and 40 μm are obtained andindividual particles may still significantly exceed this range of grainsizes. This can be attributed mostly to the high firing temperatures, aswell as to the use of fluxing agents with a strongly mineralizingeffect. Electroluminophores of this grain size class have theabove-described shortcomings.

In patent document U.S. Pat. No. 5,643,496, the process is modified tothe extent that zinc sulfide electroluminophores can be obtained thathave a grain size smaller than 23 μm, preferably 21 μm, and which, byadjusting the temperature of the first firing process to between 1100and 1190° C., preferably to 1160° C., allegedly reach the level of 25 μmlarge ZnS electroluminescent materials regarding their attainable levelsof brightness and half lives.

Such a minor reduction in the average grain size of theelectroluminophores hardly results in any noticeable improvements evenfor the use in screen printing processes. The principal shortcomings ofEL phosphor particles of this coarseness largely still exist.

Average EL pigment grain sizes in the range of 10 μm can allegedly beattained with a process according to U.S. Pat. No. 5,635,111; however,the solution described in that patent document has significant technicalshortcomings. These lie in the fact that, on one hand, the firing isperformed in a complicated vacuum apparatus in the presence of extremelyaggressive and toxic gases (halogen halides, H₂S), which involves theassociated risks if the apparatus should fail. On the other hand, thevery time consuming and expensive process hardly appears suitable forproducing larger quantities of EL luminophores under technicalconditions. The present invention is therefore based on the object ofcreating a novel, cost-effective method for producing fine-grain zincsulfide electroluminophores that can be made into efficient andlong-lived electroluminescent layers of a high quality with variousprinting techniques.

SUMMARY OF THE INVENTION

In accordance with the present invention this object is met with thetechnical teaching, of claim 1. The inventive process is characterizedaccordingly by the following preparation steps and measures:

Step 1: Preparation of special, fine-grain zinc sulfides and use ofthese materials as a starting product for the synthesis of the inventivefine-grain zinc sulfide electroluminophores.

The preparation of zinc sulfide starting materials of this type takesplace by precipitation of ZnS from the solutions of zinc salts, such ase.g., ZnSO₃, Zn(NO₃)₂ and ZnCl₃, preferably from zinc sulfate solutions,with the aid of induced H₂S gas or resulting from the addition ofH₂S-generating compounds at temperatures of 20 to 80° C. and a pHbetween 0.5 and 3.0. The zinc ion concentration of the given zinc saltsolutions is adjusted to values of 0.25 moles/I to 2.0 moles/I.

This precipitation reaction produces as a result fine-grain zincsulfides with very narrow grain-size distributions and the desiredaverage grain size can be controlled by guiding the process parameters,such as, e.g., the zinc ion concentration, the speed with which the H₂Sis passed in, the stirring speed, the temperature, and the pH.

The average grain sizes of the zinc sulfides prepared according to thisinvention and used as the starting material for the synthesis of theinventive electroluminophores are typically 2 to 20 μm, preferably 2 to5 μm or 5 to 15 μm.

A further advantage lies in the fact that, in addition to the grainsizes, the surface properties of the ZnS precipitation products can alsobe controlled via the specific selected precipitation conditions. Thisresults in compact ZnS crystallites with a very low tendency to formagglomerates. These special morphological characteristics of the ZnSstarting materials prepared according to this invention advantageouslyaffect the grain structure and the performance of the resultinginventive electroluminophores.

A further advantage lies in the fact that, in addition to the grainsizes, the surface properties of the ZnS precipitation products can alsobe controlled via the specific selected precipitation conditions. Thisresults in compact ZnS crystallites with a very low tendency to formagglomerates. These special morphological characteristics of the ZnSstarting materials prepared according to this invention advantageouslyaffect the grain structure and the performance of the resultinginventive electroluminophores.

Step 2: Mixing of the fine-grain ZnS starting materials producedaccording to this invention with the activator-coactivator compoundsrequired for the luminophore formation.

The copper and/or gold compounds, or copper and/or gold and manganesecompounds (e.g. CuSO₄, HAuCl₄, 0.4H₂O, MnSO₄) which are used asactivator materials, Au tetrachlorolaurate as well as aluminum compounds(e.g. Al(NO₃)₃) that may be required for the coactivation may already beadded during the precipitation of the ZnS or also to the washed ZnSsuspension after completion of the precipitation. This permits ahomogeneous distribution of the activators and coactivators in thepreparation mixture that is advantageous for the process of theluminophore formation, and also a close contact between the activator,coactivator and ZnS particles.

However, it is also possible to dry mix the activator and coactivatorcompounds and the zinc sulfide prepared according to this invention. Inthis case a preferred process variant consists of first homogenizing theactivator and coactivator compounds with a portion of the dried ZnS andthen blending this mixture with the remaining amount of ZnS that isrequired to ensure the desired luminophore composition.

To this mixture the fluxing agents, which are described in more detailbelow, are then added as well:

Step 3: A one to ten-hour firing of the mixture at temperatures below1000° C., preferably in the temperature range between 800 and 1000° C.in air or in an inert nitrogen atmosphere or in an atmosphere consistingof a mixture of nitrogen and 1 to 10% hydrogen in the presence offluxing agents with an only slightly mineralizing action selected fromthe compound classes of the fluorides, bromides and iodides.

After completion of the firing process, the fired product is then cooledto room temperature, subsequently washed with deionized water and thenoptionally filtered and dried.

In this manner it is effectively ensured that the average grain size andgrain size distribution of the zinc sulfide electroluminophores aftercompletion of the firing process and processing of the fired productessentially conform to the ZnS starting material prepared according tothis invention and used for the luminophore synthesis. The inventivefine-grain ZnS electroluminophores that are obtained with this processstep have typical medium grain sizes between 2 and 20 μm, preferablybetween 2 and 5 μm or 5 and 15 μm.

It is important compared to the prior art that the described firingprocess is performed at temperatures below 1000° C. and that thepresence, especially of chloride-containing or strongly mineralizingfluxing agents is completely abstained from. While it is true that theuse of fluoride and/or bromide and/or iodide-containing fluxing agentsdoes enhance the reconstruction of the ZnS lattice and the targetedinclusion of the activators required for the formation of theluminophores, their operating mechanisms are such that the grain growthcan effectively be limited in the described temperature range.

The fluxing agents that are used according to this invention can, at thesame time, function as a source for the insertion of the coactivators.For this purpose they optionally receive certain tervalent cationiccomponents (e.g. Al³⁺ Bi³⁺ in addition to the above halide anions andother cationic components.

A further advantage of the inventive process compared to the prior artties in the fact that the synthesized electroluminophores remaincompletely in their cubic crystal modification because of the firingtemperature being limited to a maximum of 1000° C. As will be describedlater, this fact results in advantages regarding the attainable levelsof brightness and half-life of the inventive electroluminophores. Inprocesses representing the prior art, hexagonally crystallizing ZnSelectroluminophores are obtained initially. These are subsequentlysubjected to an intense and often harmful mechanical milling process inorder to achieve an at least partial reverse transformation to the cubicstructure type. The related shortcomings are prevented in advance withthe present inventive process.

Step 4. Treatment of the powdery electroluminophores obtained after themain firing process with organic and inorganic acids.

The fine-grain zinc sulfide phosphors obtained after the implementationof preparation steps 1 through 4 are characterized by high photo andcathode luminescence yields. This fact is an indication of the effectiveinclusion of the activators and coactivators into the ZnS lattice aswell as of the high effectiveness of the luminescence processes thatoccur under these excitation conditions.

It needs to be noted, however, that the phosphors that have beensynthesized in this manner do not yet have optimum electroluminescentproperties.

The efficiency of the electroluminescence can be increasedsignificantly, however, if the zinc sulfide luminophore powder issubjected to a treatment with organic or inorganic acids, such ashydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃),acetic acid, or citric acid after the main firing process. For example,acid treatment can take place in about 37% HCl solution for two to sixhours while stirring at room temperature. In another embodiment, acidtreatment is with 10–20% citric acid for 4–8 hours at 60° C. withstirring.

For this purpose the zinc sulfide phosphors obtained according to thisinvention are suspended in certain quantities of the solutions of theseacids at temperatures between 20 and 60° C., preferably at roomtemperature, while stirring and the retention time of the luminophoreparticles in the given acid bath may span a range from 10 minutes up to10 hours depending upon the type and concentration of the chosen acid,as well as on the chosen temperature. The powdery ZnSelectroluminophores are subsequently filtered out, washed to pHneutrality and optionally dried at temperatures of 100 to 120° C.

As shown by electron-microscopic examinations, the inventivefinest-grain electroluminophores show a clearly changed morphology ofthe crystallites after this acid treatment. It is characterized by ahigh roughness of the crystallite surfaces as well as by the formationof grooves, corners, edges and other structural defects. Such amodification of the habit of the doped ZnS crystallites apparently is animportant prerequisite for the increase in the electroluminescenceyields of the zinc sulfide luminophores that is noted after the acidtreatment.

An advantageous secondary effect of the described acid treatment lies inthe further reduction of the average grain size of the luminophoreparticles, as well as in the further narrowing of the grain sizedistributions. The scope of this effect can be controlled via theconditions of the acid treatment. The acid treatment furthermoreenhances the de-agglomeration of the luminophore particles, resulting inadditional advantages for the use of the inventive luminophores inelectroluminescent layers (dispersion behavior, layer homogeneity).

Step 5: Re-doping of the finest-grain electroluminophores that have beensynthesized according to this invention with certain quantities ofactivator and/or coactivator ions.

Even with the inventive process it is possible to further increase theEL efficiency through the customary repeated addition of certainquantities of activator compounds, particularly Of CuSO₄ and/orcoactivator compounds, particularly those containing Al³⁺ ions, andrenewed firing of the resulting solids mixtures at temperatures between300 and 800° C. In this manner a fine adjustment of the activator and/orcoactivator concentrations and distribution of the active luminescencecenters is achieved in the ZnS matrix.

This re-doping can be effected with copper, gold, manganese, and/oraluminum compounds. Preferred compounds for this re-doping includecopper sulfate and/or tetrachloroauric acid or its sodium, salt, and/ormanganese sulfate and/or aluminum nitrate.

Re-doping can take place in an air or an inert nitrogen atmosphere, orin an atmosphere consisting of a mixture of nitrogen and 1 to 10%hydrogen.

After a firing time of preferably 30 minutes to 10 hours, the firedproduct is cooled to room temperature and subsequently washed with H₂O,mineral acids (e.g., HNO₃), or KCN solution to remove activator and/orcoactivator compounds that were not inserted into the ZnS lattice andhave precipitated on the surface.

Alternatively, the ZnS is washed with a mineral acid and then withdeionized water to pH neutrality and subsequently filtered and driedafter their treatment with mineral acids of DCN solution following there-doping.

Step 6: Annealing of the zinc sulfide electrolurninophores obtainedafter preparation step 5 for 30 minutes to 5 hours at temperaturesbetween 200 and 500° C.

Alternatively, this annealing step may take place after step 4.Annealing may take place in air or in an inert nitrogen atmosphere or inan atmosphere consisting of a mixture of nitrogen and 1 to 10% hydrogen.

This preparation step, which concludes the inventive process, serves forthe final manifesting of the luminophore composition that isadvantageous for the performance of the inventive finest-grain zincsulfide electroluminophores.

The essence of the invention thus lies in combining the describedprocess steps, particularly in carrying out the sequence of the first 4steps listed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows grain size distribution curves for zinc sulfide.

FIG. 2 shows d₅₀ values of grain size distribution ofelectroluminophores of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With this method a fine-grain cubic zinc sulfide is already generatedduring the first step, which is then used as the starting material forthe synthesis of the inventive electroluminophores, and the averagegrain size, grain size distribution and crystal structure of which isessentially maintained by limiting the firing temperature to a maximumof 1000° C. and forgoing the use of fluxing agents with a stronglymineralizing effect in step 3 of the inventive process. At the same timethe treatment of the luminophore powders obtained after the firingprocess with inorganic or organic acids according to step 4 togetherwith the process steps 5 and 6 ensures that the electroluminophoressynthesized according to this invention have all the composite andstructural characteristics required for attaining a high performancedespite their small grain sizes.

The sequence of the above 4 process steps has now made it possible forthe first time to obtain powerful fine-grain electroluminophores withgrain sizes of 2 to 20 μm by purely preparative means and in acost-effective manner without having to subsequently correct the grainsize by means of milling, and screening, which would entail seriousshortcomings.

For screen printing applications, inventive zinc sulfideelectroluminophores with average grain sizes of 5 to 20 μm are used.Electroluminophores with these dimensions can be advantageouslyprocessed into high-performance EL lamps with a significantly improvedlayer structure.

Inventive fine-grain electroluminophores with average grain sizesbetween 2 and 5 μm, on the other hand, are particularly suitable forapplications in intaglio or offset printing. They permit theimplementation of fine electroluminescent graphic structures, e.g., assecurity marks in value product printing. These particles areparticularly suitable for use in narrow-mesh screens of 120 meshes/inch.

In any case, the inventive luminophores are characterized by abrightness-service life relationship that is adapted to the givenapplication and optimal with respect to the adjusted grain size.Moreover, studies have confirmed that EL elements can be constructedusing electroluminophores with average grain sizes of 6 μm producedaccording to this invention that display levels of brightness and halflives which, under identical operating conditions, are comparable to ELfilms manufactured from commercially available coarse-grained ELpigments with grain sizes of 20 to 40 μm.

As previously mentioned, the surprisingly high service life of theelectroluminophores produced according to this invention, especially ofthose EL pigments that have average grain sizes of 2 to 5 μm inaccordance with this invention, is very likely attributable to theircubic crystal structure, which deviates from the commercially availableEL pigments.

According to the prior art this is considered advantageous for attaininghigh levels of brightness and stability.

The zinc sulfide electroluminophore particles can be coated with thinorganic and/or crystalline or amorphous inorganic protective layers soas to increase their service life further. In one embodiment, theprotective layer consists of an inner metal oxide film and an outersilicon nitrate film.

The zinc sulfide electroluminophore particles are dispersible, and canbe used for printing in a halftone photogravure ink, flexographicprinting ink, offset printing ink, letterset printing ink, gravureprinting ink. The zinc sulfide electroluminophore particles of thepresent invention can be applied onto thermal transfer films andtransferred to printable material by means of transfer printing.Alternatively, the zinc sulfide electroluminophore particles areembedded in thermoplastic granule matrices and processed into films bymeans of extrusion/coextrusion and/or thin film casting.

To further improve the stability, the individual crystallites of theinventive electroluminophores may also be provided with suitableprotective layers according to the prior art. Numerous methods andmaterials are known for applying such protective layers.

Further details and advantages of the invention will be explained belowbased on examples and drawings.

EXAMPLE 1

101 of a 1.4 molar ZnSO₄ solution are entered into a reaction vessel.The pH of this solution is subsequently adjusted to 1.0 under additionof sulfuric acid (H₂SO₄).

The precipitation of the fine-grain zinc sulfide takes place by passingH₂S gas into the prepared solution while stirring (stirring speed 700rpm). The volume flow of the H₂S gas is 36 l/h, the work is performed ata reaction temperature of 60° C.

After a reaction time of approximately 500 min. the H₂S flow is stopped.Any H₂S still remaining in the reaction vessel is exhausted, theobtained ZnS suspension is decanted, repeatedly washed with deionizedwater and finally filtered off. The obtained fine-grain ZnS powder issubsequently dried at a temperature of 120° C.

Curve 1 in FIG. 1 shows the grain size distribution of the fine-grainzinc sulfide prepared in this manner, which was determined with the aidof a Coulter counter grain size measuring instrument. What is strikingis the very narrow distribution of the ZnS grain sizes (the so-called QDvalue, which is calculated based on the equation QD=(d₇₅−d₂₅/d₇₅+d₂₅),may be regarded as a measure for the range of the distribution, which,in the present case is QD=0.134); a d₅₀ value of 4.7 μm was determinedfor the average grain size of the ZnS material prepared according to theabove described process.

In the next step a certain amount of the obtained zinc sulfide isstirred into a copper sulfate solution. After concentrating and dryingof this suspension at approximately 120° C., the material, which is nowpresent as a mixture of solids, is once again homogenized dry andsubsequently sifted with a 35 μm gauze. The weighed-in quantities of ZnSand CuSO₄ are calculated such as to establish a copper content of 1.5%for the zinc sulfide copper “activator”.

A comparable process is also used in the preparation of correspondingBiI₃ “activator”. In the example described here, the BiI₃ content of theZnS—BiI₃ mixture is 8.5%.

The preparation of the batch subsequently takes place by a thoroughblending of 1.65 kg of the fine-grain zinc sulfide, 81.5 g of the copper“activator”, 7.5 g of the ZnS—BiI₃ mixture, as well as 5.2 g aluminumfluoride (AlF₃). This mixture is entered into covered quartz pans andfired for 2 hours at a temperature of 980° C. in an N₂/H₂ atmospherewith a hydrogen content of 1.5%.

After completion of the firing process, the fired material is cooled toroom temperature and repeatedly washed with deionized water.

This is followed by the acid treatment of the obtained material. Forthis purpose the washed fired material is entered into an acid bath and2 1H₂O and 500 ml of a 37% hydrochloric acid are added relative to 1 kgof the fired material while stirring. After a retention time of onehour, this is followed by decanting and washing, with deionized water topH neutrality.

The renewed addition of copper sulfate to this aqueous suspension servesto re-dope the luminophore material. The amount of CuSO₄ used for thispurpose is calculated according to the ratio of 2 g Cu per 1 kgluminophore.

After concentration by evaporation and drying of the suspension, the drymaterial is fired in open quartz pans for 2 hours at 600° C. in air.This is followed by an acid wash with 10% HNO₃ as well as repeatedwashing with H₂O to pH neutrality. This is followed by decanting,filtering and drying.

In a concluding process step, the obtained material is once againannealed in open quartz pans for 2 hours at 300° C. in air andhomogenized by sifting after it has cooled off.

As a result of these preparation steps a ZnS—CU luminophore with a greenelectroluminescence is obtained that is characterized by a high level ofbrightness and long half-life. The average grain size of the powderyelectroluminophore is 5.2 μm (QD=0.265). As can be seen from FIG. 1(Curve 2), the average grain size of the EL pigment prepared accordingto the example is only significantly above that of the ZnS startingmaterial used in this process.

EXAMPLE 2

As in example 1, the precipitation of the zinc sulfide takes place afterH₂S gas is passed into a ZnSO₄ solution, however, the reactionparameters are adjusted differently. The reaction is started with a 0.25molar ZnSO₄ solution, the pH is fixed to 1.6, the H₂S volume flow is 60l/h and the reaction temperature is 40° C.

The zinc sulfide that is present in the suspension after completion ofthe precipitation reaction has an average grain size of 17.0 μm(QD=0.174, see FIG. 2, curve 1). The obtained ZnS suspension is washedrepeatedly with deionized water and decanted; afterwards a sufficientamount of copper sulfate is added to establish a copper concentration ofthe ZnS material of 200 ppm after the activation. The copper activatedZnS suspension is transferred to drying pans and dried at 120° C.

To prepare the starting mixture for the firing process, 1.75 kg of theactivated zinc sulfide, 0.5 g BiI₃, and 2.5 AlF₃ are thoroughly blended.The firing takes place in covered quartz firing pans at 990° C. in air.The firing time is 5 hours.

After cooling the fired product to room temperature and washing it withdeionized water, a 5-hour acid treatment is performed with 20% citricacid. This is followed by decanting and washing with H₂O to pHneutrality.

The re-doping of the luminophore material again takes place throughaddition of copper sulfate (502.5 mg per 1 kg luminophore) to theaqueous ZnS:Cu suspension.

After concentration by evaporation and drying of the suspension the dryproduct is fired in open quartz pans for 3 hours at 700° C. in air. Thisis followed by treatment with 10% HNO₃ and repeated washing with H₂O (topH neutrality), decanting, filtering and drying.

The concluding annealing of the zinc sulfide electroluminophore takesplace in open quartz pans for 1 hour at 500° C. in air, followed bycooling and sifting.

The resulting ZnS:Cu luminophore has an intense blueelectroluminescence, as well as a long half-life. As shown by Curve 2 inFIG. 2, the d₅₀ value of the grain size distribution of the inventiveelectroluminophore presented in this example, which describes theaverage grain size, is 14.5 μm (QD=0.156) and thus somewhat below thevalue determined for the corresponding ZnS starting material.

1. Zinc sulfide electroluminophores which have a cubic crystal structureand average grain sizes of from 2 to 5 microns; Wherein the particlescomprise zinc sulfide, activator compounds, and coactivator compounds,wherein the particles are coated with a protective layer consisting ofan inner metal oxide film and an outer silicon nitrate film.
 2. Zincsulfide electroluminophores comprising zinc sulfide and activatorcompounds and coactivator compounds, wherein said electroluminophoreshave an average grain size of from 5 to 7 microns and a cubic crystalstructure, and wherein particles of said zinc sulfideelectroluminophores are coated with a protective layer consisting of aninner metal oxide film and an outer silicon nitrate film.
 3. Zincsulfide electroluminophores comprising zinc sulfide, wherein saidelectroluminophores have an average grain size of from 5 to 7 microns,and wherein particles of said zinc sulfide electroluminophores arecoated with a protective layer consisting of an inner metal oxide filmand an outer silicon nitrate film.